Fluid simulation - direct access to surface possible?
You can export or access the simulation data in many ways:


obj or other mesh formats that gives you surface vertices locations per frame. Depending on your application you can use a Shrinkwrap modifier and a subdivided plane to keep the vertex count constant between frames
render Z-depth images of the surface from camera from above, giving you every visible surface point location in space (this has same limitations as that shrinkwrap trick)
you can use python and scripting to generate any custom data you want and to export it. With python api you can for example cast rays where you need to sample the surface and measure distances


One thing to note is that Blender's fluid simulation is meant for visuals, it is not meant to be scientifically accurate. You may find that in real world the fluid behaves a bit differently.